A new experimental method for the measurement of catalyst surface area of supported catalysts has been developed using selective physisorption. The desorption characteristics of a gas are studied separately on the catalyst, the support, and the supported catalyst by carrying out thermal desorption experiments in a continuous flow sorptometer. Differences in the coverage vs. temperature curves, obtained from the thermal desorption experiments, are a measure of the selectivity of the physisorbing gas, and allow calculation of the fraction of total surface area occupied by the catalyst.Two systems have been studied utilizing the thermal desorption with carbon dioxide as adsorbate: potassium carbonate/carbon black and silver/alumina. Supported catalyst surface area was determined for each system; the results were confirmed using physical mixtures of the two components (where the actual area of each component is known) and oxygen chemisorption for the silver/alumina system. The experimental technique allows for straightforward calculation of the catalyst area. D. J. MILLER and H. H. LEE Department of Chemical EngineeringUniversity of Florida Gainesville, FL 3261 1The exposed surface area of a catalyst on a support is an important parameter in the characterization of catalytic behavior, particularly in sintering and dispersion studies. Several experimental methods of determining catalyst area have been developed, including chemisorption, electron microscopy, and X-ray techniques. The difficulties involved in applying electron microscopy to actual catalysts and the sophistication of small angle X-ray scattering prevent their practical application in many cases. Chemisorption techniques have met with success in several catalyst systems, but the chemisorption techniques are limited to well-defined metal catalysts and are not in general applicable to nonmetallic catalysts. This paper presents a new method for determining catalyst area using physical adsorption. The method examines the adsorption characteristics of a gas separately on the catalyst (in powder or gauze form), the support, and the supported catalyst using a thermal desorption technique for the determination of the total exposed catalyst area as a fraction of the total catalyst and support area. This method has several advantages over other methods used in catalyst area measurements:1) The physisorption experiments are easy to carry out and the equipment needed is low cost. The physisorption experiments show good reproducibility, and sample preparation is the same as in conventional physisorption.2) The method can be applied to any catalyst system with the appropriate choice of adsorbing gas, since gases physisorb on all surfaces at low temperatures. This is in contrast to chemisorption, where the specific gas-solid interactions limit application to a few systems.3) The total catalyst area is determined directly from the experiments. Unlike chemisorption, there is no need to know the adsorption stoichiometry to determine the total area. CONCLUSIONS AND SIGNIFIC...

A rate expression is developed for the growth rate of gallium arsenide based on a postulated mechanism of the growth kinetics. This rate expression, when applied to the experimental data reported by Shaw (4), describes the growth rate quite accurately over wide ranges of temperature and concentrations. In particular, it describes in a quantitative manner the temperature and normalGaCl concentration dependence of the growth rate, which goes through a maximum with the temperature and the concentration. The growth rate as affected by diffusion is given in terms of concentration boundary layer and the intrinsic growth rate obtained. A criterion of negligible diffusional effect is developed. The effect of physical orientation of substrate surface on the growth rate is also presented. These results allow one to determine rather completely the growth rate as affected by temperature, vapor‐phase composition, fluid velocity, and the substrate orientation.

The alternating flow model (AFM) views dispersion in packed beds as a sequence of streamline plugs that must repeatedly split and merge as the bulk fluid traverses the vessel. Thus, the flow in the AFM is ordered, as opposed to the random flow implied by the Fickian analogy. For mass dispersion only, model parameters arise from a priori considerations of packing geometry. Steady state and transient data (5.6 < DJd, < 54.4, 100 < Re, < 1,000, gases and liquids) show the AFM to surpass the Fickian analogy (based on correlations for dispersion coefficients) in most cases. Further, it can describe well the radial velocity profile trends in packed beds. For heat dispersion, two additional parameters (heat transfer coefficients) arise that are not functions of packing geometry. Simple correlations for these parameters and the justifications are given. Most of the comparisons made with the literature experimental results show the AFM to be at least as good as the back-fit Fickian analogy. The AFM should be most useful for packed beds with a relatively small DJd,.

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Platinum supported on silica is used as a model supported catalyst for the purpose of demonstrating that the selective physisorption method yields the fractional catalyst surface area of supported catalysts, including metal compounds catalysts for which the method is primarily intended. The selective physisorption results with nitrous oxide as adsorbate are compared with hydrogen chemisorption results for this purpose. Experimental and theoretical refinements of the method developed earlier in our laboratory are presented that allow rather accurate determination of the catalyst surface area. The refinements also make the method effective even when the catalyst covers a small portion of the total surface area. Because of the nature of physisorption, the method should be applicable to any supported catalyst including metal compounds catalysts, provided a suitable adsorbate is used. Adsorbates other than nitrous oxide and carbon dioxide which are suitable for selective physisorption, are suggested.